UWB (Ultra Wide Band) is a radio communication scheme where a wide frequency band in bandwidth of 7.5 GHz ranging from 3.1 GHz to 10.6 GHz is used. A specification for physical layers in the UWB is standardized in IEEE 802.15.3a by IEEE (The Institute of Electrical and Electronic Engineers) working for defining standard specifications.
In the physical layer in the UWB, two modulation schemes MB-OFDM (Multi Band-Orthogonal Frequency Division Multiplexing) and DS-UWB (Direct Spread-UWB) are employed.
In the MB-OFDM, the whole band ranging from 3.1 GHz to 10.6 GHz is divided into 14 bands (frequency bands). For each of the divided bands, QPSK (Quadrature Phase Shift Keying) is conducted in primary modulation for carrying digital signals in radio waves, and OFDM (Orthogonal Frequency Division Multiplexing) is conducted in secondary modulation for achieving communications tolerant to interference from other radio waves.
In the DS-UWB, a lower band from 3.1 GHz to 4.9 GHz and a higher band from 6.2 GHz to 9.7 GHz are used. For signal transmission, the QPSK or BPSK (Binary Phase Shift Keying) is conducted in the primary modulation and CDMA (Code Division Multiple Access) is conducted in the secondary modulation. Also, a direct frequency spread scheme is used in the DS-UWB in the transmission of signals.
In the UWB, these modulation schemes are used to reduce transmission power levels. Specifically, the FCC (Federal Communications Commission) specifies that EIRP (Equivalent Isotropically Radiated Power) indicative of regulated transmission power level per 1 MHz in the UWB be less than or equal to −41.25 dBm. This level corresponds to about 0.5 mW of total power, which may be about 1/20 of total power of the PHS (Personal Handy-Phone System).
In the MB-OFDM, each of the 14 bands is assigned to a band group including two or three bands. Bands 1-3 are assigned to band group 1, bands 4-6 are assigned to band group 2, bands 7-9 are assigned to band group 3, bands 10-12 are assigned to band group 4, and bands 13-14 are assigned to band group 5.
Also, in the MB-OFDM, frequency hopping is used. The frequency hopping is a technique for communications through continuous transition of a communicating band to other bands as illustrated in FIG. 12.
Even in cases where an error occurs in communicated data due to noise having occurred in a certain frequency, the continuous transition between communicating bands could correct the error-occurring data by using data communicated in other bands.
In the MB-OFDM, patterns for frequency hopping are defined as TFC (Time Frequency Code) as illustrated in FIG. 13. In FIG. 13, TFC 1 is illustrated to have the frequency hopping pattern “BAND 1→BAND 2→BAND 3→BAND 1→BAND 2→BAND 3.” Also, TFC 5 is illustrated to use only BAND 1 without frequency hopping.
If it is not specified in upper layers which channel is used to transmit what data, the band groups are defined depending on regions in countries. On the other hand, TFCs are selected in the sequence specified in the MB-OFDM. In this manner, the band groups and the TFCs are determined, and accordingly channels are determined.
In conventional radio communication apparatuses, AGC (Automatic Gain Control) is conducted for suppressing fluctuation of the amplitude of a received electrical signal and converting it into a signal with a constant amplitude. In AGC, gain (amplification degree) is increased for a small amplitude signal based on RSSI (Received Signal Strength Indicator) while gain is decreased for a large amplitude signal.
As one implementation of AGC, Japanese Laid-Open Patent Publication 2002-94408 discloses an arrangement including a first attenuator provided between a front-end or a high frequency amplifier and a mixer, a RSSI circuit for generating a RSSI output signal from a received signal, and a second attenuator provided before the RSSI circuit for adjusting an input signal level. In this arrangement, the RSSI output signal is compared to two thresholds. If it is outside of the thresholds, the attenuation amount of the second attenuator and then the attenuation amount of the first attenuator are adjusted to substantially enlarge a dynamic range of input and output in/from the RSSI circuit and prevent saturation of the mixer, the AGC circuit, and other system circuits in strong electric fields without sacrificing modulation performance in weak electric fields.
As another implementation of the AGC, Japanese Laid-Open Patent Publication 2005-534252 discloses an arrangement including a RSSI circuit for measuring the RSSI of a received signal, an analog amplifier for amplifying the received signal, an analog-to-digital converter for converting the amplified signal into a digital signal, a digital AGC loop for determining digital gain based on the converted digital signal, and a digital-to-analog converter for setting the digital gain determined by the digital AGC loop in the analog amplifier. In a spectral spread system, this arrangement enables the system to follow a rapid tracking variation while averaging noise.
As another implementation of the AGC, Japanese Laid-Open Patent Publication 2006-229739 discloses an arrangement where gain correction is conducted corresponding to individual bands prior to AGC whenever the frequency hopping causes the bands to be switched.
As stated above, the TFC consists of band groups each including several bands, and in some TFCs, the frequency hopping is conducted through transition between the bands.
However, signal strength measurement circuits for measuring the strength of received signals, such as the RSSI circuit, may not have uniform measurement performance over the respective bands. For example, even if received signals have the same strength, a strength value for a certain band measured by the signal strength measurement circuit may be higher than a strength value for another band measured by the signal strength measurement circuit.
In conventional radio communication apparatuses as disclosed in Japanese Laid-Open Patent Publications 2002-94408 and 2005-534252, AGC operations in consideration of such measurement errors between bands in the signal strength measurement circuit are impossible.
In addition, the band switching period for the frequency hopping is specified to be relatively short, such as 312.5 ns, in the MB-OFDM specification. Thus if the MB-OFDM is applied to a conventional radio communication apparatus as disclosed in Japanese Laid-Open Patent Publication 2006-229739, particularly if analog AGC is conducted in such a conventional radio communication apparatus, the AGC operation may not be able to follow the above-stated fast band switching.